Field of the Invention
The present invention relates to a positive electrode composition for a non-aqueous electrolyte secondary battery and a method for producing the same.
Description of the Related Art
In recent years, mobile electric devices, such as VTRs, cell phones, and laptop personal computers, have spread and are miniaturized, and, as a power source for the mobile device, a non-aqueous electrolyte secondary battery, such as a lithium-ion secondary battery, is used. Further, recently, environmental problems must be dealt with and therefore, the non-aqueous electrolyte secondary battery is attracting attention as a power battery for, e.g., an electric vehicle.
As a positive electrode active material for a lithium secondary battery, LiCoO2 (lithium cobalt oxide) is generally widely employed, in which LiCoO2 is able to constitute a secondary battery at a level of 4 V. When using LiCoO2 as a positive electrode active material for a secondary battery, the resultant secondary battery achieves a discharge capacity of about 160 mAh/g, and such a secondary battery has been practically used.
Cobalt, which is a raw material for LiCoO2, is a resource that is scarce and unevenly distributed, and therefore LiCoO2 as a positive electrode active material has disadvantages not only in that the cost tends to increase, but also in that the supply of the raw material for the active material is likely to be unstable. In consideration of such circumstances, a lithium-transition metal composite oxide having a layer structure, such as nickel-cobalt-lithium manganate, which is obtained from LiCoO2 by substituting a part of Co in the LiCoO2 by another metal element, such as Ni or Mn, has been developed. As a metal element other than the above metal elements for substituting Co, for example, molybdenum is selected depending on the purpose.
International Patent Application Publication No. 02/041419 proposes a technique in which, with respect to a lithium composite oxide including nickel, cobalt, and manganese as essential components, a metal element, such as molybdenum, is introduced to the composition of the lithium composite oxide so that a composite oxide of the introduced metal element and lithium exhibits a diffraction peak, improving the heat stability of the battery in a charged state at a high capacity.
Japanese Patent Publication No. 2008-181839 proposes a technique in which, by using a composite oxide comprising lithium, nickel, cobalt, and a specific additive element as essential components, the positive electrode active material is improved in the heat stability and charge-discharge capacity. As examples of additive elements, combinations of elements, such as Nb+Mn+Al, and Mo+Mn, are specifically disclosed.
On the other hand, in accordance with various purposes, a technique for mixing a boron compound, such as boric acid, with a lithium-transition metal composite oxide, and a technique for allowing a boron compound to be present on the surface of a lithium-transition metal composite oxide have been known.
For example, Japanese Patent Publication No. 2009-146739 discloses a positive electrode active material obtained by coating composite oxide particles, such as Li1.03Ni0.77Co0.20Al0.03O2, with a boric acid compound, such as ammonium pentaborate, and subjecting the resultant particles to heat treatment. The positive electrode active material obtained as mentioned above is expected to enable a secondary battery to be increased in the capacity and improved in the charge-discharge efficiency.
Japanese Patent Publication No. 2002-164053 discloses a positive electrode active material having, on a core including a lithium compound, such as Li1.03Ni0.69Mn0.19Co0.1Al0.07Mg0.07O2, formed a surface treatment layer including a coating element, such as boron. As a coating method, specifically, a method is disclosed in which a core is treated with an alkoxide solution of a coating element and then subjected to heat treatment. The positive electrode active material obtained as mentioned above is expected to be improved in the heat stability.